The Mellen gabbro, in the Keweenawan Midcontinent Rift,

--- northern Wisconsin, U.S.A.

Figure 1.
Two views of the Mellen intrusive complex in the field. This sturdy, fresh
leucogabbro or gabbroic anorthosite is a classic plutonic rock. The coarse grain size lends an
attractive pattern to buildings constructed of cut stone, and
a low content of sulphides such as pyrite minimises rusting, and so
maintenance. Photos from Wilson (2016), taken on a field trip
organized by local geology professor Tom Fitz (Fitz, 2011).

"Rock of the Month #191, posted for May 2017" ---

The Mellen (Mineral Lake) intrusion

lies near the small college town of Ashland, not far
from the southern shore of western Lake Superior, in northern Wisconsin.
Though many may associate Wisconsin with the dairy industry and farming in general,
and the state has a limited mining tradition (unlike neighbours Michigan and Minnesota),
many rocks do appear at surface (see Dott and Attig, 2004). The area of the attractive
college town of Ashland is underlain by the Mesoproterozoic, Keweenawan Midcontinent Rift (MCR), interpreted as a large, ultimately failed rift with magmatism largely in an interval circa 1115-1086 Ma. Within this interval, there were: 1) mafic-ultramafic intrusions, with appreciable Ni-Cu-PGE mineralization in some bodies, intruded, as far as is known, 1115-1107 Ma; 2) main stage magmatism, with great volumes of magma, 1102-1092 Ma; and 3) late activity, 1092-1086 Ma.

The eruption of the Kallander Creek volcanics (basalt, andesite and rhyolite, age dated at 1108-1102 Ma) within the MCR was associated with a large central volcano. The volcanic pile was cross-cut at 1102 Ma by the Mellen intrusive complex, a suite of gabbroic rocks, granophyre and granite (Cannon et al., 1993).

Olmsted (1969) made an early, detailed study of the Mineral Lake intrusion,
a 4.5-km-thick stratiform intrusive body within the Mellen complex, emplaced at the base of the Middle Keweenawan volcanic series. He described a layered intrusion with an average composition of anorthositic gabbro. In terms of internal thicknesses, the intrusive comprises a basal chill zone (0.1%), ultramafic rocks (1%), anorthositic olivine gabbro (10%), gabbroic anorthosite and anorthosite (73%), ferrodiorite (7.7%) and granite and related rocks (8%). Local alignment of plagioclase feldspar laths is indicative of flow in a melt-crystal mush. Chilled margins and an upward compositional variation in orthopyroxene, clinopyroxene, olivine and plagioclase composition further indicate that this is a major differentiated intrusion in the rift.

Figure 2.
A hand specimen of the gabbro, with offcuts and a polished thin section.

As noted above, intrusions in the MCR represent some of the earliest Keweenawan magmatism, circa 1115-1107 Ma. There are numerous suites of hypabyssal (shallow) intrusives within the rift, especially diabase dykes and sills, as well as early mafic-ultramafic bodies. The Baraga dykes (with high TiO2 plus Cu and Pd) may be feeders to the Kallander Lake basalts. The Mellen complex, at 1102 Ma, was emplaced in a reversed-polarity paleomagnetic interval, while the much larger Duluth complex, at 1099 Ma, was intruded in a normal-polarity period (Schulz and Nicholson, 2009).

The Mellen intrusive complex, continued

Fitz (2011) has provided a good field guide to the Mellen complex. The complex
occupies an area of 380 km2. It was intruded as large sill-like bodies into the lower part of the Keweenawan volcanic sequence. The Mellen rocks were steeply tilted northwards during the compression and folding that formed the Lake Superior syncline. The complex is bimodal in composition, consistent with the continental rift setting. The system is exposed from feeder dykes through the main intrusives to the volcanics above.
Fitz points out that the four largest intrusive complexes in the MCR are the Duluth complex, the Nipigon sills, and then the Coldwell complex and the Mellen complex. There are 4 main intrusions in the Mellen complex, the Mineral Lake, Rearing Pond and Potato River intrusions, and the Mellen granite. Strong igneous lamination defined by parallel plagioclase crystals is apparent in many of the gabbroic rocks. The Mineral Lake intrusion has two parts, separated by a screen of Kallander Creek volcanics , and granophyre is abundant high in this body. The small Rearing Pond intrusion is the most mafic body, with a peridotite layer near the base, succeeded by olivine gabbro and gabbro (Fitz, 2011).

For more on the local geology of the Mellen area see also Klasner and LaBerge (1996). Regional overviews include Wold and Hinz (1982). Further dates are available on the Mineral Lake gabbro and Mellen granite (Zartman et al., 1995, 1997).

Olmsted,JF (1969) Petrology of the Mineral Lake intrusion, northwestern Wisconsin.
In `Origin of Anorthosite and Related Rocks' (Isachsen,YW editor). New York State Museum and Science Service Memoir 18, 466pp.,149-161.